a. Field of the Invention
This invention relates generally to golf ball cores, and in particular, to golf ball cores which contain a zinc salt of a short branched chain saturated monocarboxylic acid. In particular, this invention relates to a golf ball core which contains zinc di-ethylhexoate. The present invention is useful because it provides a golf ball core having an improved COR from about 0.5% to about 2% over golf ball cores composed of conventional elastomer blends.
b. Prior Art
The distance a golf ball will travel when hit by a golf club is a function of many factors, including angle of trajectory, clubhead speed and coefficient of restitution. The coefficient of restitution ("COR") is a measurement familiar to those skilled in the golf ball art. One way to measure the COR is to propel a ball at a given speed against a hard massive surface, and measure its incoming and outgoing velocity. The COR is the ratio of the outgoing velocity to the incoming velocity and is expressed as a decimal.
The COR of a golf ball is a function of both the core and the cover. The greater the contribution of the core, the lesser need be the contribution from the cover. There is no United States Golf Association limit on the COR, but the initial velocity of the golf ball cannot exceed 250.+-.5 feet/second. As a result, the industry goal for initial velocity is 255 feet/second, and the industry strives to maximize the COR without violating this limit.
Consequently, there has been a long-expressed interest in the industry for producing golf ball cores with high CORS, wherein the cover need not make as great a contribution to the core or vice versa. There has also been a long felt need in the golf ball industry to avoid impairing another characteristic of golf ball cores, compressibility, which is also a prerequisite to excellent play and good marketability.
The prior art discloses numerous cores wherein there is a tradeoff between the desirable characteristics of a high COR and compressibility. For example, U.S. Pat. No. 4,726,590 (Molitor) discloses a composition for one-piece golf ball cores having an improved COR. The core composition includes the following components: an elastomer cross-linkable with a free radical initiator catalyst, a metal salt of an alpha-acrylate or methacrylate, a free radical initiator catalyst, and a polyfunctional isocyanate. Although the maximum COR obtained with the Molitor invention is 0.815, there is decreasing compressibility with increasing concentrations of particular isocyanates.
U.S. Pat. No. 4,838,556 (Sullivan) discloses a solid golf ball having a solid core comprised of an elastomer or admixture of elastomers, at least one metal salt of an unsaturated carboxylic acid, a free radical initiator, and a dispersing agent. According to Sullivan, the core of this invention exhibits an increase in COR ranging from about 0.5 to about 2.0 percent over that of the closest prior art core which lacked a dispersing agent in its core composition. The highest COR obtained by Sullivan was 0.816. However, at this COR, the compressibility was lowest.
U.S. Pat. No. 4,852,884 (Sullivan) discloses a golf ball core formulation which incorporates a metal carbamate accelerator. The highest COR that Sullivan achieved, while still maintaining a satisfactory compressibility, was 0.807.
U.S. Pat. No. 4,844,471 (Terrence) discloses a golf ball core composition including dialkyl tin fatty acid. The highest COR attained in the invention, while maintaining a satisfactory compressibility, was 0.809.
U.S. Pat. No. 4,546,980 (Gendreau et al.) discloses a golf ball core which contains two or more free radical initiators, at least two of which exhibit a different reactivity during the curing process. According to Gendreau, golf ball core compositions containing at least two initiators, each with a different half-life (i.e, one long half life and one short half-life), yielded a golf ball product having an initial velocity of at least 0.5 feet/ second higher than the initial velocity of the same golf ball product made with only one initiator. Gendreau did not address the problem of raising the COR while retaining sufficient compressibility.
It is an object of the invention to provide a core for a two-piece golf ball which provides a high COR without sacrificing compressibility.
It is a further object of the present invention to produce golf balls exhibiting superior playability while maintaining a satisfactory COR with a large component of the COR value contributed by the core.